Ten or more years of mobile phone use can dramatically increase the risk of developing a benign tumor on the auditory nerve, according to a study conducted by the Institute of Environmental Medicine at Karolinska Institutet in Stockholm.

The Institute found that the risk of developing the tumors, known as acoustic neuromas, almost doubled for persons who started using their mobile phone at least 10 years before diagnosis. What's more, the risk increase was confined to the side of the head where the phone was usually held, according to results of the study released Wednesday.

The study of around 150 acoustic neuroma patients and 600 healthy control patients could be used to confirm long-held fears that cell phones are bad for users' health.

Researchers pointed out, however, that only analogue phones had been in use for more than a decade at the time the study was conducted, and that they could not determine if the same results would apply to the long-term use of digital phones.

The institute's report was released as part of a larger international study known as Interphone, coordinated by the World Health Organization's cancer research institute. The results of the Swedish study need to be confirmed in additional studies before final conclusions can be drawn, the researchers noted.

The results, although preliminary, are concerning. In addition to a doubling of acoustic neuroma risk for long-term cell phone users, researchers said that when they took into account the side of the head, they found that the risk was almost four times higher on the side where the cell phone was normally used.

Acoustic neuromas usually grow over a period of years before being diagnosed and occur in less than one adult per 100,000, per year, the researchers said.

The Interphone study will take into account the study on acoustic neuromas, along with a number of other types of brain cancer in assessing the risk of low-level exposure to radio frequency magnetic fields. The research is being concentrated in countries that have the longest and highest use of mobile phones, such as Sweden, the U.K., Denmark, Norway and Germany.